A partial tree based power distribution network capacitor parallel compensation computing method is disclosed. The computing method comprises the steps of obtaining information of a power distribution network; initially calculating an initial compensation scheme of parallel compensators; judging whether the partial trees need to be re-divided; dividing the corresponding partial tree; separately determining the compensation points for each partial tree; judging whether the partial trees need to be re-optimized; further optimizing the compensation points; comparing benefit investment ratio and performing circular judgment until the numbers of each compensation node and the compensation capacities of each compensation node under the maximum benefit investment ratio are output. According to the partial tree based power distribution network capacitor parallel compensation computing method, the capacitor parallel compensation sites and compensation capacities of the radiation type power distribution networks can be provided; and the line loss rate in the running process of the power distribution networks can be more effectively reduced to improve the voltage distribution in the running of the power distribution networks and to improve the economical benefit and the power supply reliability in the running of the power distribution networks, so that computing method is high in practical value.